Laser drilling speeds BGA packaging

Although BGA packaging has been around since the early 1990s, this leadless approach has only recently started to attract broad interest. There are two reasons for the sudden spurt in interest: the need for more external connections to ICs; and the availability of improved BGA technology.

The first reason for the increased popularity of BGA technology stems from the problem of the so-called "208-pin limit." When an IC requires more than that number of external connections, a peripherally leaded package must be made larger - and then requires an unacceptable amount of board or substrate real estate. If the package size were not increased, then the lead pitch would have to be reduced to a point at which soldering yields markedly deteriorate. BGA technology circumvents those problems because the connections are to the base of the package instead of around the periphery.

A second factor that has made the BGA more attractive is the availability of the plastic ball grid array (PBGA) - developed by Motorola and refined by companies such as Tessera. As illustrated in simplified form , this approach combines micro-vias with solder bumps (balls). These BGA packages can then be mounted on a PC board with a minimum total footprint . A standard infrared heater melts the solder balls, creating joints strong enough for demanding applications such as telecommunications.

Unlike their ceramic predecessors, PBGA packages can be fabricated very inexpensively. That makes them an attractive option wherever lead counts in excess of 200 are required - even for low-cost consumer applications. A typical PBGA is a multilayer laminate, with polyimide being the most popular material for the dielectric layers. As with most new high-density interconnection technologies, successful use of BGA technology requires the ability to drill micro-vias reliably through polyimide and other insulators at high speed and low cost. Those vias must be created in the substrate before die mounting and bonding.

Laser micromachining

The unique properties of lasers make them ideal tools for micromachining a wide range of materials. Indeed, lasers have long been used in the semiconductor industry for tasks as diverse as photolithography and package welding. Since intense laser light can be focused to a small spot (a few microns or less), large amounts of energy can be applied to precise locations, allowing materials to be drilled, cut, scribed, annealed, or welded.

The specific interaction between a laser beam and a processed material depends on the characteristics of the material and the wavelength of the laser. Visible and infrared laser beams, such as those from Nd: YAG (neodymium: yttrium aluminum garnet) and C[O.sub.2] lasers, can provide intense local heating. Targeted material is essentially boiled off or vaporized. Unfortunately, the intense heating often causes thermal damage (e.g. charring) to surrounding material. The extent of the damage is defined by the "heat affected zone" (HAZ). In effect, these lasers are analogous to a tiny thermal torch.

With ultraviolet lasers, the process is quite different. When a UV photon is absorbed by a material such as polyimide,...